UPSC Prelims 2008 Q36: Science & Technology – Biology
What is the pH level of blood of a normal person?
- (a) 4·5 - 4·6
- (b) 6·45 - 6·55
- (c) 7·35 - 7·45
- (d) 8·25 - 8·35
Answer: (c) 7·35 - 7·45
Explanation
Normal blood pH level
Human blood must stay within a very narrow, precisely regulated pH range for proper physiological function.
| Option | Verdict | Reason |
|---|---|---|
| (a) 4.5-4.6 | Incorrect | Far too acidic for normal blood pH |
| (b) 6.45-6.55 | Incorrect | Too acidic, below the normal range |
| (c) 7.35-7.45 | Correct | This is the normal, tightly regulated pH range for human blood, slightly alkaline |
| (d) 8.25-8.35 | Incorrect | Too alkaline, above the normal range |
Blood pH facts:
| Point | Detail |
|---|---|
| Normal range | 7.35-7.45 |
| Nature | Slightly alkaline, tightly regulated |
Answer: (c) 7.35-7.45.
Why each option is right or wrong
- (a) Incorrect. 4.5-4.6 is far too acidic for human blood.
- (b) Incorrect. 6.45-6.55 is also too acidic.
- (c) CORRECT. 7.35-7.45 is the normal, slightly alkaline blood pH range.
- (d) Incorrect. 8.25-8.35 is too alkaline for normal blood.
Key point
Normal human blood pH is tightly regulated within the 7.35-7.45 range, slightly alkaline.
Remember
The pH level of normal human blood is 7.35-7.45.
Common mistakes
- Confusing blood's slightly alkaline normal pH with a neutral or acidic value.
- Not knowing how narrow the physiologically normal blood pH range actually is.
- Mixing up blood pH with the pH of other body fluids (like stomach acid, which is much more acidic).
Exam tip
Normal human blood pH = 7.35-7.45, slightly alkaline. A tightly regulated physiological constant.
Sources
- NCERT Class XI Biology — Body Fluids and Circulation
- John E. Hall — Guyton and Hall Textbook of Medical Physiology (Regulation of Acid-Base Balance)